Solve for $k$ : $13 = -23 + k$
Add $23$ to both sides: $ \begin{eqnarray} \\ \hphantom{13 {+ 23}} &\hphantom{=}& \hphantom{k} \\ 13 &=& -23 + k \\ \\ {+23} && {+23} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 13 {+ 23} &=& k \\ \end{eqnarray} $ Simplify. $ k = 36$